The present invention relates to a polymer blend consisting essentially of from about 50% to about 95% by weight, based on the total weight of the polymer composition, of polytrimethylene terephthalate and of from about 5% to about 50% of an elastomeric polyester. The polymer blend has particular utility in the fabrication of industrial fabrics that are particularly suitable for use in rigorous environments, where the dimensional stability and mechanical properties of the fabrics are important.
Modern industrial fabrics are commonly assembled by weaving, braiding, knitting, knotting and other known methods from polymeric monofilament or multifilament yarns. It is also known from EP 802280 to assemble such fabrics from a plurality of extruded polymeric strips or panels. The chosen polymers are most frequently polyesters, copolyesters, polyamides, polyphenylene sulfides, polyphenylene oxides, fluoropolymers or polyketones. Selection of any particular polymer for a specific application will generally be dictated by the physical and mechanical properties desired, the cost of the polymer, and the prevailing environmental conditions of the end use.
The present invention is primarily concerned with a polymer blend for use in fabricating industrial fabrics intended for environments where the dimensional stability and resistance to repetitive compressive stress of the fabrics are important. The invention is thus particularly relevant to papermaking fabrics which are used to form, drain, dewater and convey a paper web as it is created within a paper making machine.
For the purposes of this application, the term xe2x80x9cfabricxe2x80x9d is taken to mean an assembly of components. The term xe2x80x9ccomponentxe2x80x9d is taken to mean any of the components from which a fabric can be assembled, such as yarns(including monofilament, multifilament, staple and spun forms) or extrusions. The components forming the fabric can be arranged by interlacing, entangling or engagement so as to form an integrated cohesive structure, such as nets, cloth, felts, textiles and the like, which are created by weaving, knitting, knotting, joining, felting, needling, spiral winding, bonding, or similar methods. Typical components include individual monofilaments, multifilaments, spiral coils, and profiled plastics extrusions such as strips, tiles or panels. These components are generally fabricated by an appropriate method such as melt extrusion, melt spinning, casting or slitting from an extruded film. The fabricated components are then joined to form an integrated cohesive structure.
In a papermaking machine, a paper web is created in three stages. In the forming section, a water based stock of papermaking components is discharged onto a moving continuous forming fabric. As the fabric conveys the stock through the forming section, it is drained and agitated to provide a somewhat self supporting wet paper web. Drainage of the stock is augmented by various stationary elements with which the forming fabric is in moving contact. The web is then transferred to the press section where a major proportion of the remaining water is removed by mechanical pressing in a series of high pressure nips between opposed press rolls. Press fabrics are used both to convey the web, and to receive expelled water. The web then passes to the dryer section, in which it is conveyed on at least one dryer fabric over a series of heated cylinders where the remaining water is removed by evaporation. The resulting paper is then calendered, slit and wound onto reels.
Papermaking fabrics must ideally possess multiple characteristics simultaneously:
(a) they must be resistant to abrasive wear caused by their passage over the various stationary elements in the paper making machine, and by contact with solids in the stock which they are to convey;
(b) they must be structurally stable, so as to function as designed within the range of stresses imposed during their use;
(c) they must resist dimensional changes in the plane of the fabric due to moisture absorption over a wide range of moisture contents;
(d) they must resist stretching under the tension imposed by the powered rolls which drive the fabrics on the machine;
(e) they must be resistant to degradation caused by the various materials present in both the fiber-water slurry and in the materials used to clean the fabrics, at the prevailing temperatures of use.
In addition, some industrial fabrics, such as press felts used in the press section of a papermaking machine, must be resistant to compaction and repetitive cyclic compressive stress.
Of the various polymers available for industrial fabrics applications, those most commonly used in paper making are:
polyesters, in particular polyethylene terephthalate (PET) and various copolymers thereof, and
polyamides, particularly polyamide-6 (also known as polycaprolactam), and polyamide-6/6 (also known as polyhexamethylene adipamide).
Although yarns and extrusions formed from both polymer types offer certain advantageous characteristics, there are essentially two difficulties associated with their use:
(i) while PET yarn generally has adequate chemical resistance and dimensional stability, and is also amenable to weaving, having good crimpability and heatsetting behaviour, its abrasion and compaction resistance is not always adequate, especially when used in fabrics for higher speed paper machines; and
(ii) although yarns of both polyamide-6 and polyamide-6/6 have adequate abrasion and compaction resistance, they do not possess adequate dimensional stability in the moisture range found in the paper making environment, and the mechanical properties of fabrics made from them are known to change.
U.S. Pat. No. 5,137,601 to Hsu discloses papermaking fabrics, in particular press felts, whose component fibers and filaments are fabricated from polypropylene terephthalate, herein after referred to as PPT. In view of the use of xe2x80x9cpropylenexe2x80x9d in the polymer name, this appears to be a polymer of terephthalic acid and 1,2-propanediol. The fabrics are alleged to have chemical resistance properties similar to polyester, and physical properties comparable to polyamide-6. There is no disclosure of suitable intrinsic viscosities for the PPT, no identification of suitable grades, no discussion of the possibility of blending PPT with a second polymer, nor are there any teachings to suggest that this polymer may be suitable to withstand repetitive cyclic compressive stress.
Best, in EP 844320, discloses monofilaments for use in paper making machine clothing whose principle component is polytrimethylene terephthalate (described by Best as PTMT, and stated to be a polymer of terephthalic acid and 1,3-propanediol). In a preferred embodiment, the PTT may be blended with up to 45% by weight of polyurethane so as to improve the abrasion resistance of the monofilaments. Best in EP 0 965 665 also discloses monofilaments of the same polyester with up to about 50% of a polyamide. There is no disclosure in either of these applications of the appropriate grade or intrinsic viscosity of a suitable PTT, nor does the disclosure teach that blending of PTT with any polymer other than either polyurethane or polyamide will improve the ability of the components to withstand repetitive cyclic compressive stresses and therefore increase the service life of the components. Specifically, there is no disclosure or suggestion that any other polymers than polyurethane or polyamide may provide satisfactory results.
Despite these innovations, and for various other reasons including the cost of the raw materials, neat polyamide yarns are still preferred for many industrial fabric applications. The term xe2x80x9cneatxe2x80x9d as used herein refers to a polymer system containing only one polymer, e.g. polyamide-6, and nothing else, other than conventional additives such as stabilizers, plastic processing aids, colourants, and inhibitors of oxidative, hydrolytic or thermal degradation. The compaction and abrasion resistance of these polyamide based yarns is useful in physically demanding applications, such as paper makers press felts, filtration fabrics, and the like. However, the fact that these polyamide yarns absorb moisture, and thus undergo physical changes in their mechanical properties, dimensions and weight when the environment of use exposes them to different moisture conditions, limits their use in moist environments where such variations often cannot be tolerated. This difficulty is discussed inter alia in U.S. Pat. No. 4,529,013, U.S. Pat. No. 4,289,173 and DE 2,502,466 which teach that fabrics including more than 50% polyamide yarns tend to grow or stretch as they absorb moisture in a wet environment, and will shrink as they dry out, thus rendering the fabric unstable on the machine.
To circumvent these dimensional stability problems, fabric manufacturers have turned to substantially more costly polyamides, such as polyamide-6/10 and polyamide-6/12, especially in instances where wet-to-dry dimensional stability is crucial. However, the cost of these polymer resins is approximately three times greater than either polyamide-6 or polyamide-6/6. Further, polyamide-6/10 and polyamide-6/12 yarns have comparatively poorer thermal properties, making them attractive for only a limited number of very specific applications.
While the moisture stability performance of polyamide-6 and polyamide-6/6 yarns has been improved, yarn manufacturers have as yet been unable to address effectively the special combination of requirements necessary for paper machine clothing applications, in particular: cost, wet-to-dry dimensional stability, and resistance to both abrasive wear and repetitive compressive stress cycling.
Polytrimethylene terephthalate, herein referred to as PTT, which is also known as poly(1,3-propanediol terephthalate), is a polymer that has recently become commercially available. PTT appears to combine a number of the mechanical properties of both polyesters and polyamides. PTT is commercially available from Shell Chemical Co. of Houston, Tex. under the trade name CORTERRA(trademark) and is stated to be the reaction product of purified terephthalic acid and 1,3-propanediol. Two grades of PTT are currently available in bulk: carpet grade, which has an intrinsic viscosity as supplied of about 0.92 dL/g, and a second grade which has an intrinsic viscosity as supplied of about 1.3 dL/g. In both cases, the intrinsic viscosity quoted is measured according to ASTM D4603-96 on the bulk resin prior to extrusion into monofilaments; the intrinsic viscosity when measured on monofilaments of extruded PTT is somewhat lower. Unless indicated otherwise, all intrinsic viscosity values quoted herein are determined by this procedure. ASTM D4603-96 requires that the intrinsic viscosity be measured on a solution in a solvent consisting of 60% by weight phenol and 40% by weight 1,1,2,2-tetrachlorethane at a concentration of between 0.2475 g and 0.2525 g per 50 ml and at a temperature of from 29.99xc2x0 C. to 30.01xc2x0 C.
The present inventors have discovered that industrial fabrics whose components are comprised of at least 80% to about 95% by weight of PTT, and from about 20% to about 5% by weight, based on the total weight of the composition, of an elastomeric polyester, are as resistant to compaction, and are able to withstand cyclic repetitive stresses as well as components fabricated from polyamide-6/10 or polyamide-6/12 while retaining their dimensional stability. The components are found to be as chemically stable as those formed from PET and are amenable to weaving when formed into monofilament or multifilament yarns. The finished PTT blend components are thus particularly suitable for use in the assembly of industrial fabrics such as press felts for papermaking machines. Components fabricated from neat PTT having an intrinsic viscosity of about 0.90 dL/g or less cannot survive exposure to about 9,000 cycles of repetitive cyclic compressive stress (under the test conditions set out below) without catastrophic failure. We have now discovered that if at least about 5% by weight of an elastomeric polyester is added to the PTT, then components formed from the resulting blend exhibit sufficient resiliency to survive exposure to at least 54,000 cycles of compressive stress. The resistance to repetitive cyclic compressive stress exhibited by components formed from a blend of PTT having an intrinsic viscosity of at least about 0.90 dL/g, and from about 5% up to about 20% of an elastomeric polyester, is at least equal to that of similar components formed from neat PTT resin having an intrinsic viscosity of about 1.3 dL/g. Components fabricated from a blend of from about 95% to about 80% by weight PTT (having an intrinsic viscosity before processing of from about 0.90 to at least 1.3 dL/g), and from about 20% to about 5% by weight based on the total composition of an elastomeric polyester appear to exhibit resistance to repetitive cyclic compressive stress that is roughly equivalent to that obtained from components formed from neat polyamide-6, polyamide 6/6 or polyamide-6/12. Thus components formed from blends of an elastomeric polyester with PTT having a bulk intrinsic viscosity in the range of from about 0.90 dL/g to at least 1.3 dL/g appear to be suitable for use in industrial fabrics intended for environments where resistance to repetitive cyclic compressive stress, abrasive wear, chemical degradation and dimensional stability under changing moisture conditions are important.